The invention is directed to a process for removal of waste water from salt bath hardening and galvanizing operations by evaporation of waste water and complete detoxification of the evaporated concentrate.
Waste waters accumulating in hardening and galvanizing operations in many cases have a considerable content of injurious materials. There may be mentioned in particular cyanides, barium compounds, nitrites and heavy metal ions. These materials must be made harmless by costly and time consuming detoxification procedures before the introduction of the waste water into the sewer system. A number of processes are known for this purpose, e.g., the detoxification of the cyanide with chlorine bleach liquor, e.g., sodium hypochlorite, or hydrogen peroxide and the precipitation of barium compounds as sulfate. This process permits a trouble-free detoxification of the waste water but generally leads to a strong saltification. Thus, in the detoxification of 1 kg of cyanide with chlorine bleach liquor depending on the concentration up to 10 kg of salt impurities are produced. Also, in the more favorable process from this point of view in which hydrogen peroxide is employed there are still formed up to 4 kg of salt impurities.
Therefore, there have been attempts to reduce this high salt impurity content by suitable measures. Thus, it was sought to reduce the consumption of water as much as possible, which is possible, e.g., by cascade connection in the quenching of construction parts in hardening operations and has led to a certain amount of success. This process, however, has the disadvantage that the amount of salt reaching the waste water still remains high despite the smaller amount of waste water. Furthermore, there have been developed processes based on evaporating salt solutions which only contain a specific salt of a definite mixture of salts (e.g., nitrite-nitrate). Through this on one hand it is possible to reduce the amount of waste water, on the other hand the recovered salt can be reused and therewith the economy of the process be improved. However, this process can only be used with such waste waters that only contain one kind of salt or a definite salt mixture, but not with complex salt mixtures.
Also in recent times there have been increasing attempts to evaporate the entire waste water and to store the remaining salt mixture which contains all the injurious materials in unchanged form in suitable deposits (below ground deposits). In these processes until now there were exclusively employed apparatuses in which the waste water, in some cases after preceding preliminary concentration, are sprayed from outside to an internally steam heated drum. The salt film is continuously scraped off. Since when poisonous components are present in waste water the residue also is poisonous, it must either be destroyed by burning or be stored in a poisonous refuse deposit. In this process also therefore the economy is small.
Therefore it was the problem of the present invention to develop an economical process for the removal of the total injurious material containing waste water of a hardening or galvanizing operation. No poisonous components in gaseous, liquid and/or solid form should remain or form thereby and to the greatest extent possible there should none result in the waste water carried off.